1. Field of the Invention
The present invention relates generally to security of negotiable documents and, more particularly, to an improved transfer film used to print magnetically-readable characters which also bleed through to the reverse side of the document to provide a verification of authenticity.
2. Description of Related Art
Banking institutions lose many millions of dollars each year through check forgery. This problem has existed every since payments by means of checks and bank drafts became common. At one time each check was processed completely by hand. Careful visual inspection was the major barrier to check forgery. Many types of "safety papers" were developed for the printing of check blanks.
These safety papers were designed to change color or otherwise indicate tampering in some visually obvious manner. Typical of these inventions is U.S. Pat. No. 1,727,912 to Snyder. That invention discloses a safety paper of a special structure that allows ink writing to bleed through to the rear surface of the check. A comparison of the rear surface with the front surface will readily reveal any alterations in the check.
Safety paper also forms the subject of more modern inventions. U.S. Pat. No. 4,496,961 to Devrient discloses a paper impregnated with chemical-filled micro capsules. The pressure of writing ruptures these capsule. The chemical contents react to form a dye image of the writing deeply embedded in the paper where it is protected from erasure. However, micro capsules are comparatively expensive to use.
Considerable inventive effort has gone into providing fraud detection for money orders and other negotiable documents that are printed by impact printers. An impact printer, like an old-fashioned typewriter, strikes a pigmented film or ribbon with a hammer in the shape of a character to be printed. This causes an area of the film or ribbon to transfer to the document and create an image of the character. U.S. Pat. Nos. 4,936,607 and 5,033,773 to Brunea et al. disclose an improved security system for impact printing.
Briefly, an inert backing layer is coated with binders, fillers, and pigment like a normal transfer film, to which is added a microencapsulated solvent plus dye. When the film is struck during the printing process, the coated layer transfers to a document surface, creating a visible image due to the pigment. The force of the impact ruptures the micro capsules allowing the solvent to carry the dye through the paper. This results in a dye image on the reverse surface of the document and a "halo" of dye around the primary pigment image on the front surface. It is virtually impossible to alter the primary pigment image and introduce a matching "halo" image. Any alterations are readily visible, thereby preventing forgeries.
However, such an advanced penetrating dye system is not sufficient. Check processing has changed considerably from the days when safety paper alone was an adequate safeguard against forgery. Today virtually all checks are encoded with Magnetic Ink Character Recognition (MICR) symbols. These symbols, usually printed along the lower edge of a check, can be optically read by a human and magnetically decoded by a machine. In the case of manually written checks, the symbols are printed onto the check when the check is returned to the bank for payment.
There has been considerable effort to perfect transfer films for the machines that print (encode) the MICR characters onto checks. Most commonly, MICR characters are printed by a impact printer. A hammer having a raised image of a desired character strikes a transfer film, which then contacts the check or other document. Because the magnetic sensing systems that "read" the MICR character are very sensitive to the magnetic properties of the image, a whole area of the film is actually transferred to the check to form a printed image. This way if the transfer film is manufactured to have proper magnetic characteristics, then the transferred area will likewise have proper characteristics.
A number of United States patents teach the making of a typical MICR transfer film. U.S. Pat. No. 3,029,157 to Sutheim et al. discloses a transfer film wherein an inert backing sheet is coated with a mixture of a grease-like material, a bodying agent (filler), a polymer, a magnetic material, and a solvent. After the solvent evaporates, impact on the film will cause the entire layer of the dried mixture to leave the backing and transfer to a document's surface, providing a magnetically readable character.
It is important that the magnetic characters also be optically readable. To this end, it is desirable to have the transferred layer have sufficient optically density. It is also important that the transferred image not smear or detach from the document surface. Many improvements of the basic transfer film design have been directed to reducing smearing or increasing optical density of the transferred image. For example, U.S. Pat. No. 5,292,593 to Talvalkar et al. represents a recent patent that seeks to improve the basic magnetic transfer film. In that invention, the transfer coating comprises a dye to enhance optical density of the image, a magnetic pigment, a primary amide, optionally an adhesive, and solvents for the coating process.
While these various inventions doubtlessly improve the quality of MICR characters and limit problems due to smearing and/or insufficient printing density, they do little to reduce the problems with forgery or check alteration. Although MICR characters are readable by humans, they are harder to read than ordinary printing and small alterations may be difficult to detect. Generally a check blank comes from a document printing company with the left hand region of the MICR line preprinted with bank, bank account and check number information. The check user adds the amount, payee and signature to the check. The first bank to receive the check encodes the right hand MICR amount field. Fraud can be committed by altering MICR figures in the bank account so that the sum is deducted from the wrong account. Fraud can also take the form of duplicating the entire check with a color copying machine so that the same check can be cashed repeatedly. In addition, the bank customer might alter the amount field after the check has been paid and returned and claim that the check had been encoded improperly.
As mentioned above, the first bank to receive the check reads the amount field and encodes the MICR amount field. But even careful visual inspection of the check may be of little avail in spotting alteration of the MICR line or color copier produced checks. When an altered check is presented for cashing, a cashier might easily fail to detect that there is a change in the bank account number or that the entire check is a fraudulent copy.